German Patent Applications P 4141954.5 and P 42202412.7 disclose the degradation of polycarbonates with monohydroxy compounds, wherein in both cases the aim is completely to decompose the polycarbonates into bisphenol structural units, naturally with the aim of re-use for new polycarbonate syntheses, optionally, by direct condensation. In contrast, the process according to the present invention proceeds, in terms of the degradation stage, only as far as the oligocarbonate stage and re-builds these into high molecular weight polycarbonates in accordance with a special polycondensation process.
Polycondensation of oligocarbonate has been disclosed in the commonly assigned German Patent Application P 4238123.1.
The process of the present invention is neither described in the prior art nor rendered obvious by it.
Solvent-free pursuant to the process according to the invention means that halogenated hydrocarbons, ketones and hydrocarbons are not used in the degradation and re-synthesis of the polycarbonate.
Low-branching pursuant to the process according to the invention means that the content of branching agents of the formula (II) ##STR2## with X=C.sub.1 -C.sub.8 alkylidene or C.sub.5 -C.sub.12 cycloalkylidene, --S--, or a single bond,
in the re-synthesized polycarbonate does not exceed a value of 75 ppm after complete saponification and HPLC determination.
Suitable monophenols for the process according to the invention include in :particular low-boiling phenols, such as, phenol itself, cresols, chlorophenols, xylenols, isopropylphenols and p-tert.-butylphenol, preferably phenol and cresols, particularly preferably phenol.
The molar ratio of starting polycarbonate (as molar weight unit) to monophenol is between 1:1 to 1:20, preferably 1:1.5 to 1:10.
The temperatures for cleavage of the starting polycarbonates with the monophenols are between 100.degree. C. and 295.degree. C., preferably between 150.degree. C. and 250.degree. C. The process is optionally performed at pressures above atmospheric in order to keep the monophenol in the liquid phase.
The catalysts are used in the process according to the invention in concentrations of 10.sup.-8 to 10.sup.-1 tool related to 1 mol of polycarbonate units, preferably in a concentration of 10.sup.-7 to 10.sup.-2 tool.
Preferred catalysts are those of the formulae (III) and (IV) ##STR3## wherein R.sub.1-4 may independently be C.sub.1 -C.sub.18 alkyls, C.sub.6 -C.sub.10 aryls or C.sub.5 -C.sub.6 cycloalkyls and X.sup.- denotes an anion for which the corresponding acid-base pair H.sup.+ +X.sup.- .rarw..fwdarw.HX has a pK.sub.B of &lt;11.
Catalysts pursuant to the process according to the invention are, for example: tetramethylammonium hydroxide, tetramethylammonium acetate, tetramethylammonium fluoride, tetramethylammonium tetraphenylborate, tetraphenylphosphonium fluoride, tetraphenylphosphonium tetraphenylborate, dimethyldiphenylammonium hydroxide, tetraethylammonium hydroxide.
A mixture of catalysts may also be used.
Polycarbonates which are recycled pursuant to the process according to the invention include homopolycarbonates and copolycarbonates made from the diphenols of the formula (I) and mixtures thereof, wherein preferred diphenols of the formula (I) include: 4,4-dihydroxydiphenyl, 4,4'-dihydroxydiphenyl sulphide, 1,1-bis-(4-hydroxyphenyl)cyclohexane, bis-(4-hydroxyphenyl)methane, 2,2-bis-(4-hydroxyphenyl)propane, 2,4-bis-(4-hydroxyphenyl)-2-methylbutane, 2,2-bis-(3-methyl-4-hydroxyphenyl)propane, 2,2-bis-(3-chloro-4-hydroxyphenyl)propane, bis-(3,5-dimethyl-4-hydroxyphenyl)methane, 2,2-bis-3,5-dimethyl-4-hydroxyphenyl)propane, 2,4-bis-(3,5-dimethyl-4-hydroxyphenyl)-2-methylbutane, 2,2-bis-(3,5-dichloro-4hydroxyphenyl)propane, 2,2-bis-(3,5-dibromo-4- hydroxyphenyl)propane and 1,1-bis-(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane.
Particularly preferred diphenols are 2,2-bis-(4-hydroxyphenyl)propane and 1,1-bis-(4,hydroxyphenyl )-3,3,5-trimethylcyclohexane.
The diaryl carbonates arising from cleavage of the polycarbonate result from the monophenols used for the cleavage and from the chain terminators used in the polycarbonates to be cleaved.
The diaryl carbonate which may optionally be added in the second stage is preferably a carbonic acid di-C.sub.6 -C.sub.14 -aryl ester. The preferred diaryl carbonate is in particular diphenyl carbonate.
It is to be ensured that the reaction components to be added, namely monophenols in the first stage and optionally carbonic acid diaryl esters in the second process stage are free of alkali and alkali-earth ions, wherein quantities of alkali or alkali-earth ions below 0.1 ppm may be tolerated. Monophenols or carbonic acid diaryl esters of such purity are obtainable by re-crystallizing, washing or distilling the carbonic acid diaryl esters or monophenols.
If the polycarbonate waste used as starting carbonates contain condensed branching agents, these are again incorporated on recondensation so that in this case branched polycarbonates are purposefully and deliberately obtained.
The process according to the invention is preferably performed in the following three stages:
In the first stage, degradation of the starting polycarbonate to the oligocarbonate takes place at temperatures of 100.degree. C. to 295.degree. C., preferably at temperatures of 150.degree. C. to 250.degree. C. In the second stage, monophenol is distilled off by applying a vacuum between atmospheric pressure and 2 mbar at temperatures between 180.degree. C. and 260.degree. C. and optionally with addition of further diaryl carbonate, preferably diphenyl carbonate. A high viscosity oligocarbonate is achieved with an M.sub.w (weight average molecular weight determined by measuring the relative solution viscosity in CH.sub.2 Cl.sub.2 or in mixtures of equal quantities by weight of phenol/o-dichlorobenzene, calibrated by light scattering) of between 8000 and 18000, and in the third stage polycondensing at temperatures between 250.degree. C. and 295.degree. C. and pressures from &lt;500 mbarto 0.01 mbar to the low-branching polycarbonates with an between 20000 and 100000, preferably between 22000 and 60000, wherein the M.sub.w is again determined as explained above for the oligocarbonates.
The OH terminal group content of the oligocarbonates obtained in tile second stage of the process according to the invention is defined as ##EQU1## and amounts between 25% and 50%.
The OH/aryl carbonate terminal group ratio of these oligocarbonates was determined by separately determining the OH terminal groups by photometry with TiCl.sub.4, and determining the aryl carbonate terminal groups by HPLC analysis of the monophenol formed after complete saponification. In these oligocarbonates, the OH terminal groups and aryl carbonate terminal groups generally together add up to 100%.
The process according to the invention may be performed both continuously and discontinuously, and namely in stirred-tank reactors, film evaporators, stirred-tank reactors in series, extruders, kneaders, simple disk reactors or high viscosity disk reactors.
The polycarbonates obtainable in accordance with the process according to the invention exhibit the normal OH terminal group contents known from the literature.
This is achieved by the low molecular weight oligocarbonates from the second stage preferably being condensed by monophenol distillation into low viscosity polycarbonates and the higher molecular weight oligocarbonates from the second stage being condensed into higher molecular weight polycarbonates.
The polycarbonates obtainable in accordance with the process according to the invention are isolated, for example, by discharging, spinning and pelletizing.
The low-branching or purposefully branched polycarbonates obtainable in accordance with the process according to the invention may have customary additives, stabilizers etc. incorporated into them in a known manner.
The polycarbonates obtainable in accordance with the process according to the invention may be processed in customary machines, for example, in extruders or injection molding machines, into any desired moldings, for example, film or sheet in a customary manner. These polycarbonate moldings may be used industrially in a known manner, for example, in electrical engineering.
The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified.